High efficiency loss modulator



Dec. 22, 1942.

F. E. TERMAN HIGH EFFICIENCY LOSS MODULATOR Filed Dec. 21, 1940 J I. 3 nF 3 fl'c/r/A a' I 6 1 0026! 4 i para;- T Maz MAr/A G FlG.2.

INVENTOR. BY flit-0:9 0

atented Dec. 22, 1942 UNETED STATES PATENT OFFICE 2,305,911 HIGHEFFIQIENCY Loss MODULATOR Frederick E. Terman, Stanford University,Calif.,

assignor to International Corporation, New York, N.

Delaware Standard Electric Y., a corporation of Application December 21,1940, Serial No. 371,094 2-,Claims. I (Cl. 179171.5)

This invention relates to modulators, and more particularly toabsorption or lossmodulators' of relatively high efficiency; a p

In the past loss modulators, that is, modulators which operate inconjunction with high frequency generators or amplifiers by varying theradio frequency losses at modulation frequencies have been proposed.However, these, modulators have in general been relatively 'low inefficien'cy.

In my prior Patent No. 2,152,753, issued April 4, 1939, is disclosed aloss modulator in which the efficiency of such a modulator is improvedby the provision of an arrangement for maintaining the anode directcurrent substantially constant.

According to my present invention the efficiency of a modulator of thetype disclosed in the above-named patent is further improved by using arectifier in the system as a losser tube, and connecting the rectifierto oppose the direct current anode supply potential of the highfrequencygenerator or amplifier tube.

A better understanding of my invention and the objects and featuresthereof may be had by the particular description of embodiments thereofwith reference to the accompanying drawing, in which:

Fig. 1 is a circuit diagram showing my invention applied to themodulation of energy supplied from an amplifier; and

Fig. 2 shows my invention applied to modulation of energy in anoscillation generator.

It has been disclosed in my prior Patent 2,152,753, to utilize a lossermodulator tube in the form of a triode connected in the output of a tubewhich may be the output of a radio frequency amplifier or an oscillationgenerator. In the present application a similar type of losser modulatorusing a. rectifier in place of the triode is disclosed.

In Fig. l is shown a tube I which may be excited by radio frequencyexciting voltage, either as the output of a radio frequency amplifier orby an oscillation generator. Tube I is provided with the tank circuit LCto which the energy is fed over a coupling condenser 2. A rectifier 3,preferably a diode, is shunted across the tank circuit LC, the anode oftube 3 being connected to a point in the tank circuit'and the cathodebeing connected to a positive potential point on the battery 4 whichfurnishes the anode direct current for tube I.

In order that the current drawn from battery 4 may be maintainedsubstantially constant, a high impedance choke coil L2 is provided inthe anode lead from battery 6. In series with Lz is provided a radiofrequency choke 5. By this choke coil arrangement the rado frequencyenergy supplied to valve I may be modulated to a reasonably highpercentage with satisfactory linearity. This linearity may be obtainedeven when a triode is used in place of the rectifier tube. I

The modulating voltage is applied from coil 6 in series with the diode,and the tank circuit LC is so adjusted that the peak radio frequencyvoltage applied to the diode under carrier conditions, that is with nomodulation signal being applied, is just greater than the direct currentbias of the cathode. In the present case where the full plate voltage isapplied to the cathode this means that the peak voltage is slightlygreater than the D. C. plate voltage for tube I.

With this arrangement most of the radiofrequency energy absorbed fromtank circuit LC by losser tube 3 is returned to the plate supply systemin the form of D. C. current in reverse polarity, and subtracts from thetotal current consumed from the battery. If the internal impedance ofthe diode is zero, and in practice the impedance is so low as to benearly negligible, all of the power absorbed by the losser tube undercarrier conditions will be returned to the system resulting in highoverall efliciency. Thus, if the modulated tube has a class C amplifierefficiency of 75%, a power input to the plate circuit of the tube 1 of1333 watts will produce 1000 watts of radio-frequency energy. Half ofthis represents the carrier output, while the remaining 500 watts istaken by the losser circuit. Assuming all of the losser power isreturned to the system, the net input required to generate 500 wattscarrier is 1333-500 or 833 watts. This represents an efiiciency of 60%.

For complete modulation, the audio-frequency modulating voltage must besufficient to vary the potential between the cathode and ground of diode3 from zero to twice the plate-supply voltage. At the same time theaudio-frequency current that flows through the diode has a peak valuevery nearly equal to the D. C. current through the losser under carrierconditions. With complete modulation this means that the modulatingpower is approximately P/2 where P is the carrier power. For the degreeof modulation m the modulating power is accordingly m P/2. This isslightly less than the amount of audio-frequency energy required forplate modulation. The overall efliciency of the system considering theinput power to be that supplied by the battery plus the audio modulatingpower, and

the output power to be the modulated wave, rises Slightly withmodulation, being substantially 69% for complete modulation of the 500watt carrier in the numerical case given above.

It is possible with this arrangement to obtain substantially linearmodulation up to greater than 90% modulation. The limit of depth ofmodulation is set by the fact that although the load impedance ofieredto the modulating voltage by the diode circuit is substantially constantfor most of the operating range, this impedance drops abruptly to a verylow value when the envelope of the modulated wave reaches zero. Thislimitation is not particularly severe, however, and actual experimentalresults have indicated less than 10% distortion with 95% modulationwithout the use of any negative feedback to improve the operation.

In designing a modulation system of this type i it is to be noted that arectifier having negligible impedance should be used. Many of therectifier tubes now on the market, as well as some triodes re-connectedas diodes, realize this ideal to a very satisfactory degree. To theextent that the diode impedance is negligible, the power losses in thediode are zero. In practical cases the losses are so small that thediode dissipation is only a secondary design factor, the primary factorbeing the voltage rating of the diode.

In Fig. 2 is shown a circuit wherein an oscillation generator is used asthe source of the radio frequency waves. In this system the tube l issupplied with a tank circuit LC connected between the anode and grid ofthe tube through a coupling condenser 2. supply for tube l is providedover a high impedance choke L2 and RF choke 5. Diode 3 is connected withits anode to a point on the tank circuit LC, and its cathode isconnected over the coupling coil 7 to the positive anode voltage fortube The output for the modulated signal is coupled by means of a coil 8to the coil of the tank circuit L.

The direct current plate The modulating system which has been heredescribed is relatively simple and is capable of virtual completemodulation. For a given carrier output the circuit requires slightlyless modulating power than a corresponding plate modulated system. Theefiiciency that can be realized is in the order of unmodulated and whenfully modulated.

Although I have shown only two examples of circuits utilizing thepresent invention, it is clear that many other circuits may be devisedwithin the scope of the present invention. It is merely necessary thatthe broad principles outlined above be adhered to in order that thedesired results of high efiiciency loss modulation be obtained.

What I consider to be my invention and upon which I desire to secureprotection is embodied in the accompanying claims.

What I claim is:

1. A modulation system comprising an electron discharge device having anoutput circuit to which oscillating power is supplied by said device,anode potential supply means for said device, a circuit including asource of modulating voltage and a rectifier connected in series, andmeans for connecting said series circuit between a point in said outputcircuit and a positive potential point of said anode potential supplymeans.

2. A modulation system comprising an electron discharge device having anoutput circuit to which oscillating power is supplied by said device. asource of anode potential for said device, means intermediate saidsource of anode potential and said electron discharge device formaintaining the direct anode current of said electron device constant, acircuit including a source of modulating voltage and a rectifierconnection in series, and means for connecting said series circuitbetween a point in said output circuit and a positive potential point ofsaid anode potential source.

FREDERICK E. TERIWAN.

